DIY RJ45 breakout

Orionn · 2018-01-31T11:27:19.105Z

I have an emonpi, with the bundled temperature sensor and pulse counter. I have made my own RJ45 breakout by just double-jumpering each wire together into two RJ45 “mechs”. I have tested this by running ethernet over each port (one at a time, of course), so I know they’re electrically fine.

When I plug this cable into my emonpi, only one will work (either the temp sensor or the pulse counter). Do I need to do something special to get them both to work, like use a pull up/down resistor or something? The breakout on the shop web page has a resistor or something in the picture, but I can’t find any diagrams or explanations on it.

The only suggestion I can find by googling is that it’s a cable length issue. My breakout cable is about 1m long, and the temp sensor and pulse counter are standard - about 1m and 0.5m, respectively. The only other thing I can think of is that I need to wire them in a loop, rather than a star - can anyone provide a pinout or advice on that?

Robert.Wall · 2018-01-31T12:29:50.741Z

Are you sure everything is connected pin-pin correctly?

When you say each works, does each work in each “port” of your expander?
The suggestion for daisy-chain wiring will only apply to the one-wire bus, which is the temperature sensor, as will concerns about reflections in the wiring. The optical sensor is just a voltage level - it’s not even been through a Schmitt trigger so it is vaguely analogue - so cable length and capacitance will only slow the pulse edges a little.

I can’t see a reason why the two devices won’t co-exist, other than a wiring fault.

I’m not sure that the Ethernet test is conclusive. Many Ethernet devices can auto-detect whether the wiring is T568A or T568B, and self-adjust accordingly.

glyn.hudson · 2018-02-01T23:22:38.619Z

Recommend testing the electrical connections directly rather than testing with Ethernet. See emonPi Wiki for RJ45 pinout EmonPi - OpenEnergyMonitor Wiki

Orionn · 2018-02-02T03:50:39.108Z

OK, it looks like a confusing mix-up. You can just double-jumper the cables 1-to-1. I think the temperature sensor cable got squashed in the door of the meter box, and it started working once the cable was un-squashed. And the optical pulse meter wasn’t working on the new power meter that was installed a week ago because I misinterpreted what the LED does.

To help someone who might be searching for this, the 2-element meter is an Atlas MK7A. “Pulse 1” LED shows kWh by both elements (e.g. anytime power and controlled-load power), and the second LED (“Pulse 2”) shows varh, which in my case never flashed. To measure the controlled load (off-peak hot water), I am measuring the pulses to calculate power from the grid, then subtracting my use for my other circuits (as measured by a CT). I have solar, but the controlled load only comes on at night, so the solar won’t affect the calculations.

Thanks for your help, though.